Complement is an important effector system in autoimmune and inflammatory tissue injury. Decay accelerating factor (DAF, CDS5) is a GPI-anchored cell surface protein that restricts complement activation on autologous cells. The objective of this proposal is to define the in vivo biology of DAF, both in preventing end organ injury as a complement inhibitor and as an emerging negative regulator of adaptive immunity. Our overall hypothesis is that DAF not only protects end organs from complement attack but also negatively regulates T/B cell responses to self and foreign antigens. Two specific aims are proposed: 1. To delineate the in vivo activity of DAF as a complement inhibitor in mouse models of end organ injury. We hypothesize that DAF functions in vivo mainly as a classical pathway (CP) complement regulator, whereas Crry, a widely expressed murine membrane C3 regulator regarded as the functional homologue of the human membrane cofactor protein (MCP, CD46), controls both the CP and the alternative pathways (AP). We postulate that Crry determines tissue sensitivity to AP complement attack, while levels of both DAF and Crry together influence tissue sensitivity to CP complement attack. We will create bone marrow chimeric mice, Crry conditional knockout mice and Crry knock-in (to the DAF locus) mice, which will be used along with the previously generated DAF knockout mice to test our hypothesis in models of complement-mediated tissue injury. These studies may shed light on the in vivo activity and relationship of DAF and MCP on human tissues. 2. To test the hypothesis that DAF is also a negative regulator of adaptive immunity in vivo and to elucidate the mechanism(s) by which DAF exerts this effect. We will use DAF knockout mice, TCR transgenic mice and complement-blocking reagents to test this hypothesis in an ovalbumin immunization and an autoimmune disease (experimental autoimmune encephalomyelitis) model. These experiments may establish a novel and critical link between complement and the adaptive immunity. Collectively, the proposed studies will increase our understanding of DAF and facilitate its therapeutic exploitation in autoimmunity, organ transplantation and vaccine development.